It’s interesting that you posed this question, because I saw something curious a couple of weeks ago, while driving. I actually saw a small bird (smaller than a Robin) fall from the sky and hit the pavement very close to my car.
Was it intercepted by a Hawk? (they are fairly numerous in my neck of the woods) Did it die a "natural" death while flying? I have no idea as to what led up to my observation, and I have to say that I never before witnessed something like that.
But, I can report that I have seen dead birds in my backyard from time to time. My best guess is that dead birds are usually eaten quickly by stray cats, dogs, foxes, etc. Or, the other possibility is that they go to Gary, Indiana to die.